Thas cool man, but he said it was a sunleaves ballast. Sunleaves switchable ballasts have a switch on them to enable or disable the ignitor. If ther is no switch, the ballast wasnt the same one as the box originally held. I am not aware of a traditional magnetic balllast that is capable of recognizing a mh vs hps lamp. There is no smart chip of some kind in them just a ballast, capacitor (and an ignitor if it runs hps.)
RM
I assume the 400 ballast is a standard coil ballast not a digital?
If ther is no switch on the ballast cover it isnt switchable.
HPS bulbs need an ignitor to get them started. mh bulbs do not except with the pulse start mh's. If your 400 was switchable it would have an ignitor in the box. THe switch on the ballast box is used to bypass the current so the ignitor isnt part of the circuit when running a mh bulb with it. The only way it will work wit hps is if you took it apart and rewired it with an ignitor. Not worth the safety factor compared to the lowcost of a 400 watt coil ballast.
If you want to keep it cheap, go with the mh bulb you have now and add some flourescent lighting to supplement with a lower kelvin rating as was mentioned earlier.
You could use a conversion bulb like the hortilux ace i think its called or the less expensive sumaster but both are almost as expensive as a new 400 watt magnetic coil ballast.
you can get 125w cfl's that are 2700 kelvin for $40 or less.
One of them would add a good amount of orange to red light and increase your yield a bit.
RM
All you need to do is check if your ballast has a ignitor. If it does then its convertabile.
FLUX CAPACITOR....HAHAHAHA... Must of came out of a DeLorean.